1. Field of the Invention
The invention is directed to the field of optical imaging of skin and in particular to a noncontact imaging system for rapidly and quantitatively characterizing skin using a digital camera incorporating crossed-polarizers and an image analysis method.
2. Description of the Prior Art
Port wine stain (PWS) birthmark is a congenital, progressive vascular malformation of the skin that occurs in an estimated 7 children per 1,000 live births and typically occur on the face and neck. Approximately 1,500,000 individuals in the United States and thirty-two million people worldwide have PWS birthmarks. Histopathological studies of PWS show a normal epidermis overlying an abnormal plexus of dilated blood vessels located in the dermis. Epidermal thickness (50-150 μm) and melanin absorption, as well as PWS blood vessel diameter (30-300 μm) and depth distribution (150-1000 μm) vary on an individual patient basis and even between different areas on the same patient.
Since most of the malformations occur on the face, PWS is a clinically significant problem in the majority of patients. PWS should not be considered a cosmetic problem but a disease with potentially devastating psychological and physical complications. Personality development is adversely influenced in virtually all patients by the negative reaction of others to a “marked” person. Detailed studies have documented lower self-esteem in such patients and problems with interpersonal relationships. Studies have indicated a high level of psychological morbidity in PWS patients resulting from feelings of stigmatization that are frequently concealed in casual social interactions. In childhood, PWS are flat red macules. Lesions tend to darken progressively to purple, and by middle age, they often become raised as a result of the development of vascular nodules. The hypertrophy of underlying soft tissue, which occurs in approximately two-thirds of lesions, further disfigures the facial features of many patients.
Historically, therapeutic approaches to treatment PWS have included ionizing radiation, dermabrasion, cryosurgery, and electrotherapy. Clinical results using these methods were unsatisfactory due to cosmetically unacceptable scarring post treatment. Pulsed dye lasers (PDL) are currently used for the clinical management of PWS patients. PDL treatment produces reasonably good results in a limited population of PWS patients due to its ability to destroy selectively dermal blood vessels. Yellow light (λ=585 nm to 595 nm wavelength) emitted by this laser is preferentially absorbed by hemoglobin (the major chromophore in blood) in the dilated PWS blood vessels where, after being converted to heat, causes thermal damage and thrombosis. PDL treatments are currently administered by moving a laser handpiece, which creates a 5-10 mm diameter spot on the skin surface, in a methodical fashion across the entire PWS such that adjacent sites are treated sequentially.
Presently, patients are treated using laser parameter selection based on clinical judgment of the physician. A number of PWS characteristics such as size, color, anatomical location, and patient age, have been considered as prognostic parameters of response to PDL therapy. To date, none of these has been accepted as a reliable predictor of therapeutic outcome. Treatment results vary in large part due to site-to-site and interpatient variability in epidermal melanin absorption, PWS depth and blood vessel size. To further complicate the picture, absorption of laser energy by epidermal melanin reduces the light dosage reaching the blood vessels, thereby decreasing the amount of heat produced in the targeted PWS.
Because PWS blanching is almost never achieved after just one treatment, additional sessions are typically required, with a three-month interval between successive patient visits. Unfortunately, if the ultimate standard required is complete blanching of the lesion, even after many repeat treatments, complete blanching is rarely achieved where an average success rate of below 10% is experienced. We believe that this occurs primarily because the attending physician is unable to select the optimal treatment parameters for a specific PWS lesion.
Pretreatment knowledge of tissue parameters on an individual patient basis can result in optimization of PWS laser therapy. However, due to a dearth of clinically accepted devices for determining these parameters, clinicians must still rely on subjective qualities such as PWS skin appearance and personal experience to determine treatment parameters (e.g., wavelength, pulse duration, spot size, light dose, and cryogen spray cooling factors) to use on each patient.
Commercial devices, such as reflectance spectrophotometers and tristimulus colorimeters, can provide quantitative information on skin erythema and melanin. In the biomedical field, these devices have been used to quantify skin color changes induced by UV radiation exposure. Erythema indices have been measured that were compared with subjective evaluation of PWS blanching provided by clinicians. Decreases in erythema indices correlated well with improved PWS blanching, demonstrating the feasibility of using these erythema metrics in management of PWS patients undergoing therapy. As the PWS is blanched, the value of the erythema metric used would approach that of the surrounding normal skin. Furthermore, if the erythema metric changes minimally between two successive visits, the clinician could alter treatment parameters or deem the patient as a nonresponder and stop treatment. This is especially important for treatment of children, as they routinely are subjected to full anesthesia prior to each treatment session; it is especially important to identify children who are unresponsive to laser therapy to avoid unnecessary anesthesia.
The basic operation of common commercial devices involves irradiation of skin with a light source and capture of reflected light using a combination of limited bandwidth photodetectors. The Dermaspectrophotometer™, reflectance spectrophotometer (Gyberderm inc. Media, Pa.) emits light at green and red wavelengths for semi-quantitative calculation of erythema and melanin indices, respectively. A single small-area photodetector detects reflected light at each emission wavelength, and erythema and melanin indices are computed. Similarly, tristimulus calorimeters illuminate the skin with white light and reflected light is detected with three filtered photodiodes sensitive to either red, green, or blue (RGB) light.
With these kinds of reflectance spectrophotometers, melanin and erythema indices are determined using one of several proposed algorithms. These algorithms typically involve algebraic expressions incorporating reflectance values measured at three or four select wavelengths. The data is subsequently converted to the Commission Internationale de I'Eclairage (CIE) L*a*b* color space (Table 1), which consists of three quantities: L* describes the reflected light intensity and varies between 0 (e.g., black) and 100 (e.g., white); a* describes color saturation and varies between −60 for green and +60 for red; and b* also describes color saturation and varies between −60 for blue and +60 for yellow. Studies have shown that L* and a* are viable indicators of melanin and erythema, respectively. Other studies have shown that b* and combinations of L* and b* are viable indicators of melanin.
TABLE 1Color range of parameters in CIE L*a*b* color spaceCIE ParametersQuantitative RangeL* (Light Intensity) 0 (Black) to +100 (White)a* (Saturation)−60 (Green) to +60 (Red)b* (Saturation)−60 (Blue) to +60 (Yellow)The L*a*b* coordinate axes are orthogonal to one another (FIG. 1). Studies have shown that a* and L* values (Table 2) represent the degree of skin erythema or hemoglobin content and the degree of skin pigmentation or melanin content, respectively.
TABLE 2Definitions of L*, a*, and Δa*ParametersDefinitionL*Indicator of melanin contentHigher value represents lowermelanin contenta*Indicator of erythema, which isdirectly related to hemoglobincontentHigher value represents highererythemaΔa*Indicator of erythema differencebetween PWS and normal skinPositive values indicate that theregion of interest has moreerythema than the referencenormal skin regions
Although reflectance measurement techniques can provide valuable information on PWS skin, they are generally limited in usefulness by practical considerations. They provide information on only a small area (˜10-15 mm in diameter) per measurement; thus, it may be time consuming to measure an entire PWS. For example, for a PWS of 100 cm2 area, characterization of the entire area would require over 100 measurements. Furthermore, since these devices are required to be in contact with skin, variations in contact pressure can induce artifacts in the measured reflectance values. Application of excessive pressure can result in transient blanching of the PWS due to blood vessel collapse, resulting in potentially large error in measured values.
A potential alternative approach that may ameliorate many of these difficulties is based on digital photography. This technology offers advantages such as computer interface for near real-time feedback, flexibility of measurement area selection, and noncontact technique. However, for a digital imaging system to provide meaningful results, parameters such as camera sensitivity, shutter speed, aperture size, magnification, and patient positioning must be controlled. Furthermore, image quality may be affected by shadowing, glare, nonuniform illumination, changes in spectral qualities of the illumination source with time and artifacts resulting from environmental lighting.
Medical imaging is critical for quality health care, yet remains unavailable to many patients in small hospitals, rural communities and underdeveloped nations. With recent developments that provide inexpensive portable computation and consumer camera systems capable of high fidelity, mega-pixel resolution imaging, it might be possible to somehow develop inexpensive, innovative, high-resolution imaging devices with emphasis on early detection and efficient treatment of disease and injury. What is needed is a low-cost, quantitative digital imaging system that can be applied in a quantitative way to assess in-vivo tissue. While we have chosen to focus on port wine stain (PWS) because of its high inherent optical contrast in the visible portion of the electromagnetic spectrum, the method of the invention will be applicable to a variety of tissues for which information related to changes in tissue composition is desired and not just PWS.